1. Field of the Invention
The present invention relates generally to an apparatus for photochemically ashing a photoresist. The apparatus is practically employed during one of a series of steps for forming semiconductor integrated circuits on base boards. More particularly, the present invention relates to an apparatus for photochemically ashing a photoresist wherein the apparatus induces uniform photochemical oxidation decomposition of the photoresist over the whole surface of each base board.
2. Description of Related Art
To facilitate understanding of the present invention, a typical conventional apparatus for photochemically ashing a photoresist will be described. FIGS. 5 and 6 illustrate such a conventional apparatus as disclosed in Japanese Laid-Open Utility Model No. 84429/1989.
The conventional apparatus is essentially composed of a hollow chamber 1 which is exposed to the outside environment by an open bottom surface. A base board 2 (e.g. a silicon wafer) is held on a circular table 3 which is arranged to rotate in the interior of the chamber 1. Inside the chamber 1, a plurality of ultraviolet ray lamps 4 are arranged to irradiate the base board 2 and a plurality of ozone feeding nozzles 5 are directed at the base board 2 to provide ozone gas in the proximity of the base board 2. The ultraviolet ray lamps 4 of the conventional apparatus emit two different wavelengths of ultraviolet rays (i.e. 253.7 nm and 184.9 nm) in the presence of an oxygen radical which induces ashing of a photoresist on the base board 2.
A disadvantage of the conventional apparatus is that localized ashing of the photoresist occurs directly below the ozone feeding nozzles 5. The time required to accomplish ashing of the remaining part of the photoresist not directly under the ozone feeding nozzles 5 is considerably longer. Consequently, an extended period of time and an excessive quantity of ozone gas is required for completely ashing the whole base board 2.
To obviate one disadvantage of the conventional apparatus, an improved conventional apparatus, shown in FIGS. 7 to 9, has been proposed by the applicant of the instant invention in Japanese Laid-Open Utility Model No. 135732/1989. As best shown in FIGS. 8 and 9, the improved conventional apparatus is characterized by an ozone feeding nozzle 7 including a slit-shaped passageway 6 flared at its end portion which is proximate to the upper surface of the base board 2. Ozone gas is supplied through the ozone feeding nozzle 7 and flows across the upper surface of the base board 2 in both a radial direction c (as shown in FIG. 7) and a circumferential direction d (as shown in FIG. 8). The improved apparatus provides a more uniform ashing than the conventional apparatus, however, excessive quantities of ozone gas are still required.
Specifically, it has been observed from operation of the improved conventional apparatus that the ozone gas is easily "peeled off" the upper surface of the base board 2 (as indicated by arrow b in FIG. 7) due to the presence of irregularities on the upper surface of the base board 2. Once peeled off, the ozone gas tends to be captured between the ultraviolet ray lamps 4 with little or no opportunity to repeatedly participate in the ashing. Consequently, an excessive quantity of ozone gas is consumed uselessly.